1. Field of the Invention
This invention relates to the field of semi-fusible links, and particularly to semi-fusible link systems used for parameter trimming in integrated circuits.
2. Description of the Related Art
"Fusible links" have been used in integrated circuits (ICs) for many years, as a means of trimming one or more IC parameters. A fusible link is characterized as having "intact" and "blown" states, providing a very low resistance when intact and an open circuit when blown. For example, an operational amplifier's offset voltage might be trimmed using a number of fusible links connected in series with a number of resistors, with the links and resistors arranged such that the resistance between the amplifier's offset input and ground can be adjusted by blowing open the appropriate links.
A number of approaches have been used to create fusible links. For example, an "anti-fuse", i.e., a link that can be driven from an "open" state to a short circuit, is created by forcing a high current through a zener junction to trigger a reverse breakdown, which melts the metal contacts together across the junction to form a short. These "zener zap" links typically operate at 25-35 volts and require currents that would be destructive to a reasonably-sized transistor made with the same process. For this reason, zener zaps are typically programmed using dedicated bond pads for each link.
Minimum width metal tracks have also been used as fusible links, with a current forced through the metal to melt the link open. As with the zener zaps, the high currents required to open such links typically require dedicated bond pads for each link.
Fusible links have also been made from polysilicon ("poly"). Poly links are superior to either zener zaps or metal links in that the power required to blow open a poly link can be provided by an active circuit device, such as a reasonably large n-channel MOSFET. The ability to blow the links with active devices removes the need for the bond pad-per-link requirement, such that many links can be programmed by, for example, a serially-loaded shift register which requires only 2 bond pads. However, poly links do require a considerable amount of chip space to accommodate the large active devices needed to provide the programming current.
In addition to the above-noted problems, the substantial currents (referred to herein as the "programming currents") needed to blow poly, metal, or zener zap links may damage the semiconductor. In particular, a die's thin glass passivation layer, which protects the circuit from mechanical or chemical damage, can rupture or crack during link blowing. This may allow moisture to penetrate to the chip's surface and allow corrosion to begin.